The present invention relates to a degaussing device and a degaussing method for color CRTs (Cathode Ray Tubes) in color video monitors, color TV receivers and the like.
Color CRTs used in color video monitors, color TV receivers and the like are generally subject to blurs or nonuniformities in display color under the effects of terrestrial magnetism or external magnetic fields (the blurs and nonuniformities will be referred to as xe2x80x9ccolor blursxe2x80x9d hereinafter). This is due to the fact that an electron beam in a CRT is affected by magnetic fields before the beam reaches the fluorescent screen thereof. This results in a positional error of the arrival point (mis-landing) of the electron beam. In order to-suppress such effects due to external magnetic fields, a magnetic shield made of an about 0.3 mm thick mild steel plate, which is called inner magnetic shield, is incorporated inside the color CRT, the magnetic shield constituting magnetic shielding in combination with a frame and a shadow mask or an aperture grille, which are soft magnetic materials. It is also practiced that a magnetic shield member of permalloy or amorphous material or the like is provided outside the color CRT, for example, along its housing.
These magnetic shielding members are magnetized by external magnetic fields applied, such as terrestrial magnetism. Since diamagnetic fields caused by the magnetized members are opposite in direction to the original magnetic fields applied from external, applied magnetic fields inside the magnetic shielding are canceled. However, if the magnetized state of the shielding members remains as it is (this state will be referred to as xe2x80x9cresidual magnetizationxe2x80x9d hereinafter), the canceling effect by diamagnetic fields can no longer be obtained when the direction of the color CRT is changed.
In order to remove the residual magnetization of the magnetic shield members, the shadow mask or an aperture grille and the like, there is provided a device for demagnetizing or degaussing by applying a damped oscillation magnetic field from outside of the color CRT.
Common degaussing devices degauss magnetic shield members within a color CRT by passing a damped oscillation current through a degaussing coil when the unit having the color CRT is powered on. However, electronic appliances having a color CRT used in ships and the like, color TV receivers for use in sightseeing buses, or the like, unlike appliances installed indoors, are subject to large changes in the direction of the color CRT in short time while the unit is being used. This gives rise to a need to perform the degaussing from time to time during the use of the unit. However, as a current flow through the degaussing coil at arbitrary timing would cause the display image to be disturbed, degaussing during the use of the unit is fulfilled by a method as shown in FIG. 9. Electric current is passed through the degaussing coil during a vertical retrace period. This vertical retrace period varies depending on the resolution of display. FIG. 9A shows a display example in VGA (480xc3x97640 pixels), and FIG. 9B shows an example of high definition display in S-VGA (600xc3x97800 pixels) or XGA (768xc3x971024 pixels) or the like. Thus, the higher the definition of display, the shorter the vertical retrace period, with the results of not only reduced degaussing effect for one passage of damped oscillation current but also a residual magnetization toward the direction of magnetic flux generated by the first peak of degaussing current.
As devices that perform the degaussing during the use of the unit as described above, there have been known (1) Japanese Patent Laid-Open Publication HEI 5-236488, (2) Japanese Patent Laid-Open Publication HEI 8-275188, and (3) Japanese Patent Laid-Open Publication SHO 59-148484.
With the degausser disclosed in the Japanese patent publication (1), in synchronization with a vertical retrace pulse of a vertical output signal, damped oscillation magnetic flux is generated by a degaussing coil and the direction in which the damped oscillation magnetic flux is generated is inverted alternately. With this arrangement, there will be prevented the residual magnetization toward the direction of magnetic flux generated by the first peak of degaussing current. However, it is disclosed that the inversion cycle of the direction in which the damped oscillation magnetic flux is generated is set to a time period of from several seconds to several tens of seconds. Polarity inversion at such intervals would cause the residual magnetization to persist for at least some inversion cycles, so that color blurs during the period would not be eliminated. With the degausser disclosed in the Japanese patent publication (2), an alternating voltage is applied to the degaussing coil once every about 1 second in synchronization with the vertical synchronizing signal, and each time the alternating voltage is applied, the polarity of the alternating voltage is inverted. With such a method, however, damped oscillation magnetic flux could not be generated by the degaussing coil, and moreover because of long intervals of polarity inversion, color blurs that would occur during those intervals would be discerned. With the degausser disclosed in the Japanese patent publication (3), a single pulse in phase with the vertical synchronizing signal is generated every other vertical synchronizing signal, and the polarity of the pulse is inverted alternately every vertical retrace period while a damped oscillation current is generated with its amplitude decreasing, by which degaussing is fulfilled. With this method, there would occur color blurs within a period lasting until the damped oscillation current is damped enough, and those color blurs would be discerned. For example, in the case of a vertical synchronizing signal having a frequency of 60 Hz, since one circle period is about 16 ms, the process in which color blurs are eliminated would be discerned over a period of 160 ms on condition that, for example, damping is iterated 10 times before completion.
An object of the present invention is to provide a degaussing device and a degaussing method for color CRTs which have solved the above-mentioned problems and in which a current flow through the degaussing coil is effected during the vertical retrace period so that any disturbance of display images is prevented and that color blurs in halfway stages during the degaussing process are made indiscernible.
In order to achieve the object, the present invention provides a degaussing device for color CRTS which comprises degaussing-current passing means for passing a first damped oscillation current within a vertical retrace period of a signal fed to a color CRT, which starts with a specified polarity and swings both positively and negatively through a degaussing coil of a color CRT, and passing a second damped oscillation current opposite in polarity to the first damped oscillation current through the degaussing coil of the color CRT; and timing control means for making both the first damped oscillation current and the second damped oscillation current sequentially passed for the same times within such a short time as color blurs cannot be discerned and during a vertical retrace period.
FIGS. 1A and 1B show examples of the relationship between a vertical retrace period Tr and the first and second damped oscillation currents. FIG. 1A shows an example in which the first damped oscillation current and the second damped oscillation current are sequentially passed in two vertical retrace periods Tr adjacent to each other on the time base. By passing damped oscillation currents opposite in polarity to each other through the degaussing coil in two vertical retrace periods within such a short time as color blurs in the degaussing process cannot be discerned, as exemplified by two vertical retrace periods adjacent to each other on the time base, there will be canceled the residual magnetization due to the biasing of the damped oscillation magnetic flux toward one direction. Moreover, because of a short time interval between the passage of the first damped oscillation current and the passage of the second damped oscillation current, color blurs during the interval therebetween cannot be discerned. In particular, when the first damped oscillation current and the second damped oscillation current are sequentially passed in two vertical retrace periods adjacent to each other on the time base, the residual magnetization due to the biasing of the damped oscillation magnetic flux toward one direction is canceled in the shortest time, so that the time required for degaussing is reduced to a large extent.
FIG. 1B shows an example in which the first damped oscillation current and the second damped oscillation current are sequentially passed within one vertical retrace period Tr. By passing damped oscillation currents opposite in polarity to each other through the degaussing coil within a vertical retrace period, the residual magnetization due to the biasing of the damped oscillation magnetic flux toward one direction is canceled. Moreover, since the damped oscillation magnetic flux is generated only during the vertical retrace period, there do not occur color blurs in the degaussing process themselves.
The present invention also provides a degaussing device for color CRTS as described above, wherein the degaussing-current passing means comprises capacitors connected in series to both ends of the degaussing coil respectively, switch devices provided between both ends of the capacitors and a common potential respectively, and a DC power supply connected to one-side ends of the two capacitors respectively, and wherein the timing control means turns the switch devices ON or OFF in synchronization with a vertical synchronizing signal.
Thus, by connecting capacitors in series to both ends of the degaussing coil and selectively passing filled charges of either one of the two capacitors, it becomes possible to change over the first damped oscillation current and the second damped oscillation current to each other in vertical retrace periods within such a short time as color blurs in the degaussing process cannot be discerned, or within one vertical retrace period, even if the charging of the capacitors takes a long time.